Made available in DSpace on 2014-06-11T19:22:26Z (GMT). No. of bitstreams: 0 Previous issue date: 2008-01-29Bitstream added on 2014-06-13T18:49:05Z : No. of bitstreams: 1 marques_ha_me_prud.pdf: 1098327 bytes, checksum: 37aabdd60dc21333c03db5177953ea93 (MD5) / Dados de receptores GPS de dupla freqüência são, em geral, processados utilizando a combinação ion-free, o que permite eliminar os efeitos de primeira ordem da ionosfera. Porém, os efeitos de segunda e terceira ordem, geralmente, são negligenciados no processamento de dados GPS. Nesse trabalho, esses efeitos foram levados em consideração no processamento dos dados. Foram investigados os modelos matemáticos associados a esses efeitos, as transformações envolvendo o campo magnético da Terra e a utilização do TEC advindo dos Mapas Globais da Ionosfera ou calculados a partir das pseudodistâncias. Numa outra investigação independente, os efeitos residuais de primeira ordem da ionosfera, resultantes da dupla diferença da pseudodistância e da fase da onda portadora, foram considerados como incógnitas no ajustamento. Porém, esses efeitos residuais foram tratados como pseudo-observações, associados aos processos aleatórios random walk e white noise e, adicionados ao algoritmo de filtro de Kalman. Dessa forma, o modelo matemático preserva a característica de número inteiro da ambigüidade da fase, facilitando a aplicação de algoritmos de solução da ambigüidade, que no caso desse trabalho, utilizou-se o método LAMBDA. Para o caso da consideração dos efeitos de segunda e terceira ordem da ionosfera, foram realizados processamentos de dados GPS envolvendo o modo relativo e o Posicionamento por Ponto Preciso. Os resultados mostraram que a não consideração desses efeitos no processamento dos dados GPS pode introduzir variações da ordem de três a quatro milímetros nas coordenadas das estações. / Data from dual frequency receiver, in general, are processed using the ion-free combination that allows the elimination of the first order ionospheric effects. However, the second and third order ionospheric effects, generally, are neglected in the GPS data processing. In this work, these effects were taken into account in the GPS data processing. In this case, it was investigated the mathematical models associated with the second and third order effects, the transformations involving the Earth magnetic field and the use of TEC from Ionosphere Global Maps or calculated from the pseudoranges. The first order ionosphere residual effects, resulting from pseudorange and phase double difference, were taken into account as unknown in the adjustment. However, these effects were treated as pseudo-observables and it was associated with the random process random walk and white noise and added to the Kalman filter algorithm. Therefore, the mathematical model preserves the phase ambiguity “integerness”, facilitating the application of ambiguity resolution approaches, which in the case of this work, it was used the LAMBDA method.

Global Positioning System (GPS) networks provide an opportunity to study the dynamics and continuous changes in the ionosphere by supplementing ionospheric measurements which are usually obtained by various techniques such as ionosondes, incoherent scatter radars and satellites. Total electron content (TEC) is one of the physical quantities that can be derived from GPS data, and provides an indication of ionospheric variability. This thesis presents a feasibility study for the development of a Neural Network (NN) based model for the prediction of South African GPS derived TEC. The South African GPS receiver network is operated and maintained by the Chief Directorate Surveys and Mapping (CDSM) in Cape Town, South Africa. Three South African locations were identified and used in the development of an input space and NN architecture for the model. The input space includes the day number (seasonal variation), hour (diurnal variation), sunspot number (measure of the solar activity), and magnetic index(measure of the magnetic activity). An attempt to study the effects of solar wind on TEC variability was carried out using the Advanced Composition Explorer (ACE) data and it is recommended that more study be done using low altitude satellite data. An analysis was done by comparing predicted NN TEC with TEC values from the IRI2001 version of the International Reference Ionosphere (IRI), validating GPS TEC with ionosonde TEC (ITEC) and assessing the performance of the NN model during equinoxes and solstices. Results show that NNs predict GPS TEC more accurately than the IRI at South African GPS locations, but that more good quality GPS data is required before a truly representative empirical GPS TEC model can be released.

This thesis presents my research on the development of a neural network (NN) based global empirical model of the ionospheric F2 region peak electron density using extended geophysically relevant inputs. The main principle behind this approach has been to utilize parameters other than simple geographic co-ordinates, on which the F2 peak electron density is known to depend, and to exploit the technique of NNs, thereby establishing and modeling the non-linear dynamic processes (both in space and time)associated with the F2 region electron density on a global scale. Four different models have been developed in this work. These are the foF2 NN model, M(3000)F2 NN model, short-term forecasting foF2 NN, and a near-real time foF2 NN model. Data used in the training of the NNs were obtained from the worldwide ionosonde stations spanning the period 1964 to 1986 based on availability, which included all periods of calm and disturbed magnetic activity. Common input parameters used in the training of all 4 models are day number (day of the year, DN), Universal Time (UT), a 2 month running mean of the sunspot number (R2), a 2 day running mean of the 3-hour planetary magnetic index ap (A16), solar zenith angle (CHI), geographic latitude (q), magnetic dip angle (I), angle of magnetic declination (D), angle of meridian relative to subsolar point (M). For the short-term and near-real time foF2 models, additional input parameters related to recent past observations of foF2 itself were included in the training of the NNs. The results of the foF2 NN model and M(3000)F2 NN model presented in this work, which compare favourably with the IRI (International Reference Ionosphere) model successfully demonstrate the potential of NNs for spatial and temporal modeling of the ionospheric parameters foF2 and M(3000)F2 globally. The results obtained from the short-term foF2 NN model and nearreal time foF2 NN model reveal that, in addition to the temporal and spatial input variables, short-term forecasting of foF2 is much improved by including past observations of foF2 itself. Results obtained from the near-real time foF2 NN model also reveal that there exists a correlation between measured foF2 values at different locations across the globe. Again, comparisons of the foF2 NN model and M(3000)F2 NN model predictions with that of the IRI model predictions and observed values at some selected high latitude stations, suggest that the NN technique can successfully be employed to model the complex irregularities associated with the high latitude regions. Based on the results obtained in this research and the comparison made with the IRI model (URSI and CCIR coefficients), these results justify consideration of the NN technique for the prediction of global ionospheric parameters. I believe that, after consideration by the IRI community, these models will prove to be valuable to both the high frequency (HF) communication and worldwide ionospheric communities.

This thesis creates a basic framework and provides the information necessary to create a more accurate description of the topside ionosphere in terms of the altitude variation of the electron density (Ne) over the South African region. The detailed overview of various topside ionospheric modelling techniques, with specific emphasis on their implications for the efforts to model the South African topside, provides a starting point towards achieving the goals. The novelty of the thesis lies in the investigation of the applicabilityof three different techniques to model the South African topside ionosphere: (1) The possibility of using Artificial Neural Network (ANN) techniques for empirical modelling of the topside ionosphere based on the available, however irregularly sampled, topside sounder measurements. The goal of this model was to test the ability of ANN techniques to capture the complex relationships between the various ionospheric variables using irregularly distributed measurements. While this technique is promising, the method did not show significant improvement over the International Reference Ionosphere (IRI) model results when compared with the actual measurements. (2) Application of the diffusive equilibrium theory. Although based on sound physics foundations, the method only operates on a generalised level leading to results that are not necessarily unique. Furthermore, the approach relies on many ionospheric variables as inputs which are derived from other models whose accuracy is not verified. (3) Attempts to complement the standard functional techniques, (Chapman, Epstein, Exponential and Parabolic), with Global Positioning System (GPS) and ionosonde measurements in an effort to provide deeper insights into the actual conditions within the ionosphere. The vertical Ne distribution is reconstructed by linking together the different aspects of the constituent ions and their transition height by considering how they influence the shape of the profile. While this approach has not been tested against actual measurements, results show that the method could be potentially useful for topside ionospheric studies. Due to the limitations of each technique reviewed, this thesis observes that the employment of an approach that incorporates both theoretical onsiderations and empirical aspects has the potential to lead to a more accurate characterisation of the topside ionospheric behaviour, and resulting in improved models in terms of reliability and forecasting ability. The point is made that a topside sounder mission for South Africa would provide the required measured topside ionospheric data and answer the many science questions that this region poses as well as solving a number of the limitations set out in this thesis.

Ionospheric phase delays of radio signals from Global Positioning System (GPS) satellites have been used to compute ionospheric Total Electron Content (TEC). An extended Chapman profle model is used to estimate the electron density profles and TEC. The Chapman profle that can be used to predict TEC over the mid-latitudes only applies during day time. To model night time TEC variability, a polynomial function is fitted to the night time peak electron density profles derived from the online International Reference Ionosphere (IRI) 2001. The observed and predicted TEC and its variability have been used to study ionospheric in°uence on Radio Astronomy in South Africa region. Di®erential phase delays of the radio signals from Radio Astronomy sources have been simulated using TEC. Using the simulated phase delays, the azimuth and declination o®sets of the radio sources have been estimated. Results indicate that, pointing errors of the order of miliarcseconds (mas) are likely if the ionospheric phase delays are not corrected for. These delays are not uniform and vary over a broad spectrum of timescales. This implies that fast frequency (referencing) switching, closure phases and fringe ¯tting schemes for ionospheric correction in astrometry are not the best option as they do not capture the real state of the ionosphere especially if the switching time is greater than the ionospheric TEC variability. However, advantage can be taken of the GPS satellite data available at intervals of a second from the GPS receiver network in South Africa to derive parameters which could be used to correct for the ionospheric delays. Furthermore GPS data can also be used to monitor the occurrence of scintillations, (which might corrupt radio signals) especially for the proposed, Square Kilometer Array (SKA) stations closer to the equatorial belt during magnetic storms and sub-storms. A 10 minute snapshot of GPS data recorded with the Hermanus [34:420 S, 19:220 E ] dual frequency receiver on 2003-04-11 did not show the occurrence of scintillations. This time scale is however too short and cannot be representative. Longer time scales; hours, days, seasons are needed to monitor the occurrence of scintillations.

This thesis presents an update to the ionospheric propagation factor, M(3000)F2, global empirical model developed by Oyeyemi et al. (2007) (NNO). An additional aim of this research was to produce the updated model in a form that could be used within the International Reference Ionosphere (IRI) global model without adding to the complexity of the IRI. M(3000)F2 is the highest frequency at which a radio signal can be received over a distance of 3000 km after reflection in the ionosphere. The study employed the artificial neural network (ANN) technique using relevant geophysical input parameters which are known to influence the M(3000)F2 parameter. Ionosonde data from 135 ionospheric stations globally, including a number of equatorial stations, were available for this work. M(3000)F2 hourly values from 1976 to 2008, spanning all periods of low and high solar activity were used for model development and verification. A preliminary investigation was first carried out using a relatively small dataset to determine the appropriate input parameters for global M(3000)F2 parameter modelling. Inputs representing diurnal variation, seasonal variation, solar variation, modified dip latitude, longitude and latitude were found to be the optimum parameters for modelling the diurnal and seasonal variations of the M(3000)F2 parameter both on a temporal and spatial basis. The outcome of the preliminary study was applied to the overall dataset to develop a comprehensive ANN M(3000)F2 model which displays a remarkable improvement over the NNO model as well as the IRI version. The model shows 7.11% and 3.85% improvement over the NNO model as well as 13.04% and 10.05% over the IRI M(3000)F2 model, around high and low solar activity periods respectively. A comparison of the diurnal structure of the ANN and the IRI predicted values reveal that the ANN model is more effective in representing the diurnal structure of the M(3000)F2 values than the IRI M(3000)F2 model. The capability of the ANN model in reproducing the seasonal variation pattern of the M(3000)F2 values at 00h00UT, 06h00UT, 12h00UT, and l8h00UT more appropriately than the IRI version is illustrated in this work. A significant result obtained in this study is the ability of the ANN model in improving the post-sunset predicted values of the M(3000)F2 parameter which is known to be problematic to the IRI M(3000)F2 model in the low-latitude and the equatorial regions. The final M(3000)F2 model provides for an improved equatorial prediction and a simplified input space that allows for easy incorporation into the IRI model.

The South African Global Navigation Satellite System (GNSS) network of dual frequency receivers provide an opportunity to investigate solar cycle effects on ionospheric Total Electron Content (TEC) over the South Africa region by taking advantage of the dispersive nature of the ionospheric medium. For this task, the global University of New Brunswick Ionospheric Modelling Technique (UNB-IMT) was adopted, modified and applied to compute TEC using data from the southern African GNSS Network. TEC values were compared with CODE International GNSS services TEC predictions and Ionosonde-derived TEC (ITEC) measurements to test and validate the UNB-IMT results over South Africa. It was found that the variation trends of GTEC and ITEC over all stations are in good agreement and show pronounced seasonal variations with high TEC values around equinoxes for a year near solar maximum and less pronounced around solar minimum. Signature TEC depletions and enhanced spikes were prevalently evident around equinoxes, particularly for a year near solar maximum. These observations were investigated and further discussed with an analysis of the midday Disturbance Storm Time (DST) index of geomagnetic activity. The residual GTEC – ITEC corresponding to plasmaspheric electron content and equivalent ionospheric foF2 and total slab thickness parameters were computed and comprehensively discussed. The results verified the use of UNB-IMT as one of the tools for ionospheric research over South Africa. The UNB-IMT algorithm was applied to investigate TEC variability during different epochs of solar cycle 23. The results were investigated and further discussed by analyzing the GOES 8 and 10 satellites X-ray flux (0.1 – 0.8 nm) and SOHO Solar Extreme Ultraviolet Monitor higher resolution data. Comparison of UNB-IMT TEC derived from collocated HRAO and HARB GNSS receivers was undertaken for the solar X17 and X9 flare events, which occurred on day 301, 2003 and day 339, 2006. It was found that there exist considerable TEC differences between the two collocated receivers with some evidence of solar cycle dependence. Furthermore, the daytime UNB TEC compared with the International Reference Ionosphere 2001 predicted TEC found both models to show a good agreement. The UNB-IMT TEC was further applied to investigate the capabilities of geodetic Very Long Baseline Interferometry (VLBI) derived TEC using the Vienna TEC Model for space weather monitoring over HartRAO during the CONT02 and CONT05 campaigns conducted during the years 2002 (near solar maximum) and 005 (near solar minimum). The results verified the use of geodetic VLBI as one of the possible instruments for monitoring space weather impacts on the ionosphere over South Africa.

The use of ray tracing in the analysis of certain daytime ionograms recorded at Grahamstown is discussed in this thesis. A computer program has been modified and used to trace rays in the frequency range 1 - 30 MHz. Vertical, short distance oblique, and long distance oblique ionograms have been synthesized from the results and compared with experimental ionograms for Grahamstown, the Alice - Grahamstown transmission path (64 km), and the SANAE - Grahamstown transmission path (4470 km) respectively. Ray paths have been calculated and related in detail to the models of the ionosphere and geomagnetic field. The main features of the vertical and short distance oblique ionograms can, in general, be reproduced using spherically stratified ionosphere models with electron density profiles derived from vertical ionograms. A suitable model for the geomagnetic field is a tilted dipole equivalent to the actual field at Grahamstown. The two-hop mode is shown to be, usually, the lowest on the long distance oblique records. The ionosphere model is the principal limiting factor in reproducing such ionograms, and the most satisfactory results have been those obtained with a model in which electron density is assumed to vary linearly with latitude between the profiles at SANAE and Grahamstown. The promising results obtained by ray tracing with normal ionospheric conditions indicate that the method has further possibilities which could usefully be explored. / KMBT_363 / Adobe Acrobat 9.54 Paper Capture Plug-in

Resumo: Atualmente, o GNSS, em especial o GPS, é uma das tecnologias mais utilizadas para realizar posicionamento. Os modelos funcionais relacionados com as observações GNSS são mais conhecidos do que os modelos estocásticos, visto que o desenvolvimento destes últimos é mais complexo. Normalmente, no posicionamento GNSS são utilizados modelos estocásticos numa forma simplificada, com um modelo padrão, o qual assume que todas as medidas das observações GNSS têm a mesma variância e são estatisticamente independentes. Porém, atualmente os modelos estocásticos relacionados ao GNSS vêm sendo pesquisados com maior profundidade, por exemplo, considerando efeitos de cintilação ionosférica. Este efeito pode ser considerado na modelagem estocástica já que atualmente receptores GNSS permitem a extração de parâmetros de cintilação ionosférica. Além dessa, outro tipo de modelagem estocástica pode ser realizada, no caso, trata-se da consideração da variação dos ângulos de elevação dos satélites durante o rastreio dos dados. Sendo assim, nessa pesquisa foram desenvolvidos e analisados esses dois casos de modelagem estocástica, tanto no posicionamento relativo, quanto no absoluto (por ponto). No posicionamento relativo, ao se considerar a modelagem estocástica em função da cintilação ionosférica, os resultados atingiram melhorias em torno de 93,0% em relação à modelagem padrão. No processamento e análise foram utilizados dados GPS coletados no Norte da Europa, os quais estão sob condições de cintilação ionosférica. No posicionamento relativo considerando a modelagem estocástica em função dos ângulos de elevação dos satélites, as melhorias foram em torno de 89,2%. No caso do posicionamento por ponto, as melhorias em relação a modelagem estocástica padrão atingiram valores de aproximadamente 45,1% e 42,1% considerando, respectivamente... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: Nowadays, the GNSS, especially the GPS, is one of the most used techniques to accomplish positioning. The functional models related with the GNSS observables are more known than the stochastic models, considering that the development of the last ones is more complex. Usually, they are used in a simplified form, as the standard model, which assumes that all the GNSS observable have the same variance and are statistically independent. However, the stochastic models are being investigated with more property, for example, considering the ionospheric scintillation effects. This effect can be considered in the stochastic modelling since now receivers GNSS allow the extraction of ionospheric scintillation parameters. Besides that, others stochastic modelling can be accomplished, e.g. considering the variation of the satellites elevation angles during the data tracking. Thus, in this dissertation it was investigated the two cases of stochastic modelling cited above, either in the relative or in the absolute positioning... (Complete abstract click electronic access below) / Orientador: Paulo de Oliveira Camargo / Coorientador: João Francisco Galera Monico / Banca: Mauricio Alfredo Gende / Banca: Silvio Jacks dos Anjos Garnés / Mestre

We used extensive measurements made by the Jicamarca Unattended Long-Term Investigations of the Ionosphere and Atmosphere (JULIA) and Incoherent Scatter Radar (ISR) systems at Jicamarca, Peru during geomagnetic quiet conditions to determine the climatologies of lunar tidal effects on equatorial vertical plasma drifts. We use, for the first time, the expectation maximization (EM) algorithm to derive the amplitudes and phases of the semimonthly and monthly lunar tidal perturbations. Our results indicate, as expected, lunar tidal effects can significantly modulate the equatorial plasma drifts. The local time and seasonal dependent phase progression has been studied in much more detail than previously and has shown to have significant variations from the average value. The semimonthly drift amplitudes are largest during December solstice and smallest during June solstice during the day, and almost season independent at night. The monthly lunar tidal amplitudes are season independent during the day, while nighttime monthly amplitudes are largest and smallest in December solstice and autumnal equinox, respectively. The monthly and semimonthly amplitudes decrease from early morning to afternoon and evening to morning with moderate to large increases near dusk and dawn. We also examined these perturbation drifts during periods of sudden stratospheric warmings (SSWs). Our results show, for the first time, the enhancements of the lunar semimonthly tidal effects associated with SSWs to occur at night, as well as during the day. Our results also indicate during SSWs, monthly tidal effects are not enhanced as strongly as the semimonthly effects.